The present application relates generally to the use of liquid desiccants to dehumidify an air stream entering a cooling tower. More specifically, the application relates to a cooling system construction that operates using a 2- or 3-way liquid desiccant mass and heat exchanger that can dehumidify an air stream entering a cooling tower, wherein the desiccant is absorbing moisture from the air stream in such a way that the cooling tower experiences a much higher temperature drop than is normally the case, and wherein the desiccant is subsequently regenerated using a waste heat source, which—if available—can be waste heat from the building itself, to which cooling is provided.
Datacenters are an example of buildings that contain a large amount of equipment that generates a large amount of sensible heat. Other examples include semiconductor manufacturing facilities, plastics processing facilities, industrial facilities, and other buildings where large internal sensible heat loads need to be dissipated. Datacenters typically do not have a large number of people in their space, so there is typically no need to bring in a lot of outside air, and therefore the outside air (which in other buildings can be as much as 60% of the overall heat- and moisture-load of a building) does generally not constitute a large load for a datacenter and neither is there a large humidity (latent) heat-load in the datacenter itself. Oftentimes the sensible heat that is generated in these buildings by computers and the like is rejected to a chilled water or cooling water loop that is connected to a central chiller facility, which in turn rejects its heat to a cooling tower. The problem with cooling towers is that in hot, humid climates, the cooling tower is unable to evaporate a lot of water and thus the temperature drop in the cooling water is not very large. This means that either the cooling tower has to be oversized or other means of heat rejection have to be employed. Most of the heat in a datacenter is rejected to a chilled water loop and some is rejected to the air in the datacenter which is replenished with outside air. Datacenters in effect use a lot of electricity and reject the heat that the electrical consumption generates to a chiller plant and eventually to a cooling tower. It could be very desirable if the datacenter's waste heat could be used for other purposes, in particular if the heat could be used for more efficient cooling of the datacenter itself.
Liquid desiccants have been used parallel to conventional vapor compression HVAC equipment to help reduce humidity in spaces, particularly in spaces that require large amounts of outdoor air or that have large humidity loads inside the building space itself. Humid climates, such as for example Miami, Fla. require a lot of energy to properly treat (dehumidify and cool) the fresh air that is required for a space's occupant comfort. Liquid desiccant systems are however not very common on datacenters and the like, simply because datacenters have large sensible loads internally and not large latent loads, nor do datacenter use large amounts of outside air. However, the cooling towers that support a datacenter do have large latent loads since they take in outside air. It would therefore be desirable to supply these cooling towers with dry air to improve their efficiency.
Liquid desiccant systems have been used for many years and are generally quite efficient at removing moisture from an air stream. However, liquid desiccant systems generally use concentrated salt solutions such as ionic solutions of LiCl, LiBr, or CaCl2 and water. Such brines are strongly corrosive, even in small quantities, so numerous attempts have been made over the years to prevent desiccant carry-over to the air stream that is to be treated. In recent years efforts have begun to eliminate the risk of desiccant carry-over by employing micro-porous membranes to contain the desiccant. An example of such as membrane is the EZ2090 poly-propylene, microporous membrane manufactured by Celgard, LLC, 13800 South Lakes Drive Charlotte, N.C. 28273. The membrane is approximately 65% open area and has a typical thickness of about 20 μm. This type of membrane is structurally very uniform in pore size (100 nm) and is thin enough to not create a significant thermal barrier. It has been shown that these membranes are effective in inhibiting desiccant carry-over.
Liquid desiccant systems generally have two separate components. The conditioning side of the system provides conditioning of air to the required conditions, which are typically set using thermostats or humidistats. The regeneration side of the system provides a reconditioning function of the liquid desiccant most often using heat, so that it can be re-used on the conditioning side. Liquid desiccant is typically pumped between the two sides through a heat exchanger so as to prevent a large heat load from the regenerator on the conditioner.
There thus remains a need to provide a cooling system for datacenters and other buildings with high heat loads, wherein the datacenter's internally generated heat could be used for a more efficient cooling of the datacenter itself.